OLREX

OLREX is a real time regular expression editor that can be used for learning and testing various regular expressions. It uses the Javascript core (which is borrowed from the powerful Perl regular expression) and supports all the regular expression-related functions: match(), replace(), exec(), test(), search() and split().

Each function has built-in documentation and there's also an extensive documentation that describes regular expression syntax.

This app doesn't require any special permission and works without internet connection.